1. Field of the Invention
The present invention relates to a stethoscopic sphygmomanometer and, more particularly, to a stethoscopic sphygmomanometer capable of recognizing the tapping sounds of arterial blood flow, or so-called Korotkoff sounds, in distinction from noise.
2. Description of the Related Art
In the stethoscopic measurement of blood pressure, pressure applied from an inflated cuff to a blood vessel is varied while, at the same time Korotkoff tapping sounds from the blood vessel are recognized to determine systolic and diastolic pressures.
A filter and comparator system is an approach heretofore proposed for the recognition of Korotkoff sounds out of an output signal of a microphone, which is fitted on an inflated cuff.
As generally accepted in the art, the spectral distribution of Korotkoff tapping sounds is distinguishable in frequency distribution from physical movements and externally derived noise. The filter and comparator system is derived from the above distinction and constructed such that a signal sensed by a microphone which is attached to an inflated cuff is passed through a filter to reduce the amplitude of any frequency component other than Korotkoff sounds and, then, the frequency component of Korotkoff sounds is compared with a predetermined threshold by a voltage comparator so as to determine a relationship therebetween.
However, difficulty has been experienced with the filter and comparator system for various reasons in extracting only Korotkoff sounds which is free from noise. Namely, the frequency component of Korotkoff sounds is dependent not only upon the individual but also upon the time of the measurement, cuff pressure and other various conditions of measurement. The frequency component of noise extends over a wide range, i.e., several tens to several hundreds of hertz overlapping the frequency component of Korotkoff sounds and, in addition, the waveform pattern of noise is in many cases analogous to that of Korotkoff sounds.
Another problem with the filter and comparator system is that Korotkoff sounds cannot be easily distinguished from pulse sounds when the frequency component of the former is comparatively low and, because the discrimination is performed in terms of level, the accuracy of measurement is apt to be affected by differences in the amplitude of Korotkoff sounds.
While another recognition system which is based on the waveform of Korotkoff sounds (pattern recognition system) is also known in the art, it still fails to readily identify Korotkoff sounds in distinction from noise because Korotkoff sounds and noise resemble each other with regard to waveform pattern.
Especially, immediately after the recognition of a Korotkoff tapping sound by pattern recognition, noise which is lower in amplitude than and similar in pattern to Korotkoff sounds, i.e., tailing or hangover of Korotkoff sound is sometimes recognized as a Korotkoff sound. This is ascribable partly to the fact that the amplitude of Korotkoff sounds depends upon the individual, and partly to the fact that with regard to the same person the amplitudes of Korotkoff sounds just after the start of measurement and just before the end of measurement are sometimes far smaller than the largest amplitude reached during the course of measurement. The tailing of Korotkoff sounds tends to become longer as the amplitude of Korotkoff sound becomes greater while the duration of such tailing depends upon the amplitude of Korotkoff sound, adding to the difficulty of noise removal.